Patent GB-325317-A (GB)
Title: Improvements in and relating to cathodes for thermionic devices
IPC: H01J9/04
Filed: 1928-12-04
Abstract: 325,317. Freedman, P. Dec. 4, 1928. Vacuum. tubes.- A cathode for a thermionic device comprises a metallic core coated electrolytically with a layer consisting principally or wholly of an alkaline earth metal carbonate, or carbonates. The core may be of platinum, tungsten, tantalum, molybdenum, nickel, iron or cobalt, or of tungsten, tantalum or molybdenum coated with platinum, iron, nickel or cobalt, or of tungsten, tantalum or molybdenum coated first with platinum and then with nickel, iron or cobalt. It may alternatively be of ferro-nickel, preferably an alloy of 36 per cent of nickel and 64 per cent of iron, or of this alloy coated with nickel, iron or cobalt. The core is connected as cathode during deposition of the coating. The electrolyte may be prepared by suspending finely ground alkaline earth metal carbonate or carbonates in methylated spirit containing 5-15 per cent of water and passing carbon dioxide through the liquid. The excess of suspended matter is preferably allowed to remain in order to maintain the strength of the solution during electrolysis. A voltage of 100-300 is employed. Electrophoretic deposition of the coarser particles may be prevented by use of an intermittent current, this also minimizing the detrimental effects of gas liberation. If the suspension is sufficiently fine, the use of intermittent current is not necessary. Such a fine suspension may be obtained by preparing a solution of one or more alkaline earth metal hydroxides in methylated spirit and slowly passing carbon dioxide through it. Instead of methylated spirit there may be used as solvent methyl or ethyl alcohol alone or mixed with acetone, amyl acetate or ether, or acetone alone, or pyridine alone or admixed with other solvents; it is essential that the solubility of the carbonates in the solvent should be small. A two-eompartment cell may be employed; in this case, the catholyte may consist of a solution of carbon dioxide and an alkaline earth metal compound, e.g. a solution of bromides or iodides and carbon dioxide in acetone or a solution of nitrates or chlorides and carbon dioxide in alcohol. The deposit may be mechanically strengthened after removal of the core from the bath by heat treatment in an atmosphere of carbon dioxide, or it may be provided with a protective coating of pyroxyline, cellulose acetate wax, or other substance removable during thermal treatment in the thermionic device. The coating may be applied by dipping in a solution or in the molten substance. It may alternatively be applied by incorporating the strengthening substance in the electrolyte and may in this case be partially disseminated through the main coating. An aniline dye may be deposited along with pyrovyline which is destructible to about the same temperature to enable observation of the point at which the deposit is removed during subsequent heat treatment ;examples of such dyes are Brilliant Green and Malachite Green. A catalyst may be incorporated in the deposit of the alkaline earth metal compound to assist in activation. Suitable catalysts are palladium, calcium, cadmium, lanthanum, cerium, thorium and uranium. Palladium may be incorporated by forming the main coating in a porous state and immersing in a solution of palladium chloride, the adherent solution being reduced during subsequent heating. Palladium may also be deposited by electrolysis of palladium chloride solution. Thorium oxide may be. deposited by electrolysis along with the main coating by incorporation thorium nitrate in the electrolyte, the cell being divided by a porous partition. According to the first Provisional Specification, the coating may be effected by electrophoresis of a colloidal dispersion of the carbonate or carbonates in a wax which is electrically conductive when molten such as carnauba, condellila, chinese insect wax or japan wax, or a mixture thereof, or a mixture of such waxes with non-conductive waxes such as paraffin wax or natural or artificial resins. The dispersion may be effected by grinding in a colloidal mill. The coating may also be effected mechanically by drawing the core first through a mixture of the coating substances and then through a drying furnace and repeating the process several times; a catalyst may be embodied in the coating mixture.